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+# 16-bit CPU design doc
+
+## Features
+16-bit data
+16-bit address width
+16-bit instruction width
+
+MISC
+4-bit opcodes
+
+instruction format
+xxxx xxxx xxxx xxxx - xxxx xxxx xxxx xxxx
+
+General purpose registers:
+
+    000: Zero register
+    001: RA
+    010: RB
+    011: RC
+    100: RD
+
+Special registers: MAR (memory address), IR (instruction), FR (flag)
+
+Serial peripheral ports:
+
+    101: PA
+    110: PB
+
+for communicating with the outside world.
+
+Expansion port that connects directly to the main bus
+
+Addressing mode: indirect absolute (addresses are stored in registers and point to a complete address)
+
+## Operations
+
+### Memory operations
+Format: [ 4 opcode . 3 rt . 3 rx . 6 unused ]
+
+    ldr [rx] [rt]   # load value from memory at address (register value) rx to rt
+    str [rx] [rt]   # store value in rt to memory address (register value) rx
+
+### Immediate operations
+Format: [ 4 opcode . 3 reg . 9 imm ]
+
+    ldi [rt] [imm]          # load immediate value imm to rt
+
+### Nullary operations
+Format: [ 4 opcode . 12 unused ]
+
+    hlt              # halt
+
+### Arithmetic and logic operations
+Format [ 4 opcode . 3 rt . 3 rl . 3 rr . 3 alu S ]
+
+    alu [rl] [rr] [rt] [S] # compute rl ? rr where ? is the operation defined by
+                           # 0SSS in the ALU, and store result in rt
+                           # see datasheet of 74382 and ALU impl
+    als [rl] [rr] [rt] [S] # compute rl ? rr where ? is the operation defined by
+                           # 1SSS in the ALU, and store result in rt
+                           # see datasheet of 74382 and ALU impl
+
+Note: mul, div, gt, lt, eq, neq are implemented as pseudoinstructions in the assembler
+
+### Jump
+Format: [ 4 opcode . 3 rx . 9 unused ]
+
+    jmp [rx] # Jump to address in register rx
+
+### Branch
+Format: [ 4 opcode . 1 unused . 2 flag . 9 addr ]  
+Branch if flag is set (where flag 0b00 = Z, 0b01 = C, ...)
+
+    br [flag] [addr]
+
+Flags:
+- C = ALU carry bit
+- O = ALU overflow bit
+- Z = ALU zero bit
+- S = ALU sign bit
+
+Interrupt lines: 1 (IRQ)